Turn up the thermostat to cool down blood pressure

William Rogers shows the setting on the thermostat in his house in the Dorchester neighborhood of Boston, Massachusetts March 5, 2013. Rogers, who cannot work because of his cancer treatments, keeps the heat set low to make the heating oil last as long as possible and receives fuel assistance through Action for Boston Community Development (ABCD). That fuel assistance relies on federal funding, funding which may be cut under sequestration. REUTERS/Brian Snyder (UNITED STATES - Tags: BUSINESS POLITICS SOCIETY)

Blood pressure may be affected both by the temperature of someone’s personal environment and by the number of daylight hours, researchers found.

Among patients referred to hypertension clinics and undergoing ambulatory blood pressure monitoring, each increase of 1° C (1.8º F) in daytime personal-level environmental temperature was associated with a 0.14-mm Hg drop in average daytime systolic blood pressure, according to Pietro Amedeo Modesti, MD, PhD, of the University of Florence in Italy, and colleagues.

And each 1-hour increase in daylight -- a measure of seasonality -- was associated with an increase in average nighttime systolic blood pressure by 0.63 mm Hg (95 percent CI 0.37 to 0.90), the researchers reported in the April issue of Hypertension: Journal of the American Heart Association.

Both relationships were statistically significant only for those patients who were receiving antihypertensive treatment.

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The findings may have clinical implications, Modesti and colleagues wrote.

“The negative effect of air temperature exerted in the short term is combined with the positive effects on blood pressure of the progressive increase in daylight hours from winter to summer. This pattern clearly gives reasons for the risk of marked blood pressure reduction when early increase in air temperature ensues in spring, whereas risks progressively reduce during the progression of summer,” they wrote.

“The need of a more frequent use of ambulatory blood pressure monitoring in monitoring the antihypertensive treatment in elderly patients under conditions of unstable and often extreme temperature exposures is, thus, further supported by our data, especially in the light of the debate on the possible effects on health of current world climate change,” they continued.

Previous studies have shown that blood pressure is related to ambient air temperature and seasonality -- with higher blood pressure during the winter than in the summer -- but no studies have examined its relationship with personal-level environmental temperature, which takes into account the fact that people spend most of their time inside in controlled environments.

The current study included 1,897 patients who were referred to Italian hypertension clinics and who underwent ambulatory blood pressure monitoring accompanied by a measurement of personal-level environmental temperature. The mean age of the patients was 63 and 66 percent were receiving antihypertensive drugs.

In a multivariate analysis, there were several associations between blood pressure and personal-level environmental temperature and seasonality.

In addition to the relationship between daytime systolic blood pressure and daytime temperature and between nighttime systolic blood pressure and additional daylight, there was an association between 24-hour temperature and 24-hour systolic blood pressure, but only among individuals older than 65 -- blood pressure was 0.60 mm Hg lower for every increase of 1° C.

That “indicates that health interventions targeted at better protection against cold weather (e.g., improved home heating and reduced to cold climate) may be particularly effective in the elderly,” the authors wrote.

“The main mechanisms by which exposure to high temperatures reduce average blood pressure in populations probably reflect short-term physiological adaptations to the temperature with arteriolar vasodilation and reduced peripheral resistance,” they noted.

“On the contrary,” they added, “the positive relationship between daylight hours and nighttime blood pressure suggests the potential contribution of other environmental factors.”

The number of daylight hours was also related to the morning blood pressure surge -- the surge was 0.54 mm Hg lower for each additional hour of daylight, although the relationship was statistically significant only among those taking antihypertensive drugs.

The authors acknowledged that the study was limited by the cross-sectional design and by the assessment of different individuals at various times of the year, which muddies the assessment of the relationship between air temperature and blood pressure.

In addition, all of the individuals were referred to a hypertension clinic and thus were not representative of the general population.